When an IGBT is turned ON, an electric current flows based on a hole current and an electron current. The hole current flows based on holes injected from a collector. The electron current flows based on electrons injected from an emitter. To achieve a low ON-voltage, there is a need to increase the amount of holes and electrons. The low ON-voltage can be achieved by increasing the amount of holes injected form the collector. However, when the amount of the injected holes is large, a tail current occurs due to the holes during switching so that fast switching cannot be achieved. Therefore, to achieve a low ON-voltage and a fast switching, it is very important to increase the amount of the electron current when the IGBT is turned ON. In an IGBT, the amount of injected electrons depends on the density of holes near an emitter. Therefore, to achieve a low ON-voltage and a fast switching, it is important to increase the density of holes near the emitter without excessively increasing the amount of holes injected from the collector.
However, in an IGBT, the density of holes decreases with the distance to the emitter due to diffusion and recombination. As a result, the amount of injected electrons decreases.
The present inventors consider that the above disadvantage can be overcome by forming a thin n-type layer, called the carrier storage (CS) layer, in an emitter layer. Further, a non-patent document 1 (M. Takei et al. Proc. ISPSD'10, pp. 383-386, June 2010) discloses that an oxide layer is formed in a drift layer of a vertical IGBT to narrow a hole path so that conductivity modulation can be increased.
However, the CS layer may degrade a breakdown voltage and increase a manufacturing cost. Further, the non-patent document 1 discloses a structure for increasing the conductivity modulation in a vertical IGBT only. In other words, the non-patent document 1 is silent on a lateral IGBT.